This invention relates to communication devices and in particular to optical communication devices.
Optical communications system receivers need high sensitivity and fast response. One approach to selecting an optical receiver for a communications system is to first identify receivers of sufficient sensitivity, and then to pick the receiver with the fastest response among them. Such a procedure has led the free space communications industry to select silicon avalanche photodiodes (APDs) for long distance, free space laser communications system operating at a wavelength of about 0.8 mm. However, the response time of ADPs is generally limits them to signals at a GHz. The speed of the APDs is limited by a time constant associated with the electron multiplication process. Much faster speeds are achievable using p-i-n photodiodes. However, the p-i-n photo diode sensitivity is not sufficient for many applications. It is known that the sensitivity of a p-i-n photo diode-based photo receiver can be increased by adding an optical preamplifier ahead of the photo diode to first amplify the optical signal prior to photo detection. The use of optical preamplifiers for laser receivers in fiber optics communications systems is known. The preamplifiers themselves typically consist of semiconductor lasers with reduced facet reflectivities, operated just below the lasing threshold. These semiconductor laser amplifiers do a good job of amplifying the signal, producing amplifications on of up to 25 to 30 db. However, the amplifiers are very noisy. Interference filters have been proposed to cut down on the amplifier noise with only limited success.
It is known that atomic line filters can be used to obtain ultra narrow band optical filtering. Faraday filters are also know and have been used to provide ultra narrow-band optical filtering. See Menders et al, "Ultra narrow line filtering using a Cs Faraday filter at 852 nm" (OPTICS LETTERS, Vol. 16, No. 11, Jun. 1, 1991).
What is needed is an optical communication system having transmitting and receiving components in which a signal beam which can be allowed to diminish to extremely low levels and still be detected by the optical receiving components in the system.